Molybdenum nitrosyl complexes and their application in catalytic imine hydrogenation reactions

Article abstract of DOI:10.1002/ejic.201000973

The reaction between [Mo(CO)₄(NO)(ClAlCl₃)] and the sterically hindered diphosphanes (P∩P) 1,3-bis(diisopropylphosphanyl)propane (dippp, a), 1,2-bis(diisopropylphosphanyl)ethane (dippe, b), 1,1′-bis(diisopropylphosphanyl)ferrocene (d

Full text of DOI:10.1002/ejic.201000973

FULL PAPER
DOI: 10.1002/ejic.201000973
Molybdenum Nitrosyl Complexes and Their Application in Catalytic Imine
Hydrogenation Reactions
Alexander Dybov,[a] Olivier Blacque,[a] and Heinz Berke*[a]
Keywords: Molybdenum / Hydrogenation / Imines / Hydrides
The reaction between [Mo(CO)4(NO)(ClAlCl3)] and the steri-
cally hindered diphosphanes (PʝP) 1,3-bis(diisopropylphos-
phanyl)propane (dippp, a), 1,2-bis(diisopropylphosphanyl)-
ethane (dippe, b), 1,1Ј-bis(diisopropylphosphanyl)ferrocene
(dippf, c) and 1,2-bis(dicyclohexylphosphanyl)ethane (dcype,
d) produced the chlorides [Mo(PʝP)(CO)2(NO)Cl] (1a–1d),
which were transformed into the corresponding hydrides
[Mo(PʝP)(CO)2(NO)H] (2a–2d) by reaction with LiBH4 in
Et3N at room temperature. The molybdenum–THF complex
[Mo(dippp)(CO)2(NO)(THF)][BArF4] [3a; ArF = 3,5-(CF3)2-
C6H3], obtained by the reaction of 2a with [H(Et2O)2][BArF4],
ion by the more stable [B(C6F5)4]– anion greatly increased the
catalytic activity. The use of in situ mixtures of the hydrides
2a–2d and [H(Et2O)2][B(C6F5)4] improved the hydrogenation
activity. The hydride 2b in combination with [H(Et2O)2]-
[B(C6F5)4] exhibited the highest TOF value of 123 h–1 in the
reduction of PhCH=N(α-naphthyl). The hydrogenation of the
imines PhCH=NPh, p-ClC6H4CH=NPh, p-ClC6H4CH=N-p-
C6H4Cl, PhCH=NCH(Ph)2 and PhCH=NMes showed TOF
values of 34, 74, 41, 18 and 84 h–1 at room temperature
and a H2 pressure of 30 bar. A mechanism for the ionic hydro-
genation with “proton-before-hydride transfer” is antici-
pated.
was exemplarily tested in the hydrogenation of the imine
–
PhCH=N(α-naphthyl). Replacement of the [BArF
]
counter-
4
istic studies supported the “ionic hydrogenation” pathway
with proton-before-hydride transfer.[4] Furthermore, in re-
Introduction
Homogeneous hydrogenation reactions are crucial in the lated systems, Kubas and co-workers demonstrated the cru-
production of numerous fine chemicals. Wilkinson- or Os- cial influence of phosphane substitution on hydrogen acti-
born-type catalysts[1] are normally used in reactions involv- vation through their electron-donating capability.[21–24] For
ing the formal homolytic splitting of H2.[2] These types of example, the complexes [Mo(CO)(iBu2PCH2CH2PiBu2)2]
catalytic transformations are particularly suited to the hy- bearing strong electron-donating ligands can split the H2
drogenation of olefinic compounds. “Ionic hydrogenation”, molecule at ambient temperature to form a dihydride
operating by the heterolytic cleavage of H2, is largely a new Mo(H)2 species, whereas the structurally similar [Mo(CO)-
approach.[3,4] It involves the transfer of a hydride (H–) or (Ph2PCH2CH2PPh2)2] complex coordinates the hydrogen
proton (H+) as a H2 equivalent to the substrate.[5] Various molecule with an elongated H–H bond. In isoelectronic cat-
catalytic systems enable the efficient hydrogenation of ionic bis(diphosphane)nitrosylmolybdenum and -tungsten
ketones[6–8] and imines[9–13] by Wilkinson/Osborn-type or complexes the H2 ligand adds oxidatively to form dihydride
ionic hydrogenation. However, most of these processes are complexes.[25] H2 complexes are normally more acidic than
presently based on precious metals, which require tedious the corresponding dihydride complexes. Therefore the for-
catalyst recycling for economic and toxicity reasons. There- mer species are preferably involved in heterolytic splitting
fore a global research approach has been initiated to de- by deprotonation and the latter foster catalyses of the Wilk-
velop non-precious metal catalysts.[14,15] Bullock and co- inson type. The presence of nitrosyl ligands is expected to
workers reported some molybdenum and tungsten com- reduce the binding strength of H2 ligands and enhance their
plexes [Cp(CO)2M(L)]+[A]– {M = Mo, W; L = phosphane, acidity thus promoting the heterolytic splitting of H2.[26,27]
carbene; A = BArF [ArF = 3,5-(CF3)2C6H3], B(C6F5)4} In addition we would expect for the deprotonated dihydro-
4
that showed catalytic activity in ketone hydrogenation,[16–20] gen complexes the weakening of the resulting metal–hydride
but the TOF values achieved were still quite low. Mechan- bonds to facilitate hydride transfers onto unsaturated com-
pounds.[28,29]
Based on this we supposed that (diphosphane)nitrosyl
[a] Anorganisch-Chemisches Institut, Universität Zürich,
complexes of the type [Mo(NO)(PʝP)(CO)2L][A] (L = la-
bile ligand) might render catalytic ionic hydrogenation sys-
tems, particularly for imine hydrogenations with proton-be-
fore-hydride transfer characteristics.
Winterthurerstrasse 190, 8057 Zürich, Switzerland
Fax: +41-(0)44-635-68-03
E-mail: hberke@aci.uzh.ch
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000973.
652
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 652–659

Molybdenum Nitrosyl Complexes
are in-plane with the metal. The average Mo–P bond length
is 2.5678(5) Å.
Results and Discussion
Synthesis and Characterization
Treatment of the chloride 1a with 5 equiv. of LiBH4 in
Et3N at room temperature resulted in the formation of the
hydride complex 2a (Scheme 1).[25] The 31P{1H} NMR
spectrum of 2a exhibits a singlet at δ = 41.1 ppm, which
indicates the equivalence of the phosphane coordination. In
addition to multiplets attributed to the resonance of the
dippp protons, a characteristic triplet for the hydride ligand
The addition of 1 equiv. of the ligand dippp [dippp =
1,3-bis(diisopropylphosphanyl)propane] to a THF solution
of [Mo(CO)4(NO)ClAlCl3] revealed the formation of the
chloride compound [Mo(dippp)(NO)(CO)2(Cl)] (1a;
Scheme 1), which was isolated in 75% yield. The 31P{1H}
NMR spectrum of 1a exhibits a single resonance at δ =
21.5 ppm, which indicates the presence of two equivalent
phosphorus atoms. The 1H NMR spectrum of 1a shows
two multiplets at δ = 2.36 and 2.27 ppm, attributed to the
–CH protons of the isopropyl groups. In the 13C{1H} NMR
spectrum the characteristic signal of the CO ligands ap-
is observed in the 1H NMR spectrum at –2.28 ppm (2JPH
=
24.0 Hz). In the 13C{1H} NMR spectrum of 2a the doublet
of doublets at δ = 226.0 ppm (2JCP = 10.7 Hz) has been
attributed to the carbonyl ligands. The IR spectrum of 2a
reveals strong bands at 1983, 1910 [ν(CO)], 1653 [ν(MoH)]
and 1570 [ν(NO)] cm–1.
pears as a doublet of doublets at δ = 215.9 ppm (2JCP
=
An X-ray diffraction study was carried out on 2a (Fig-
ure 2). Similar to the chloride compound 1a, the hydride
compound 2a displays a pseudo-octahedral coordination
geometry with the phosphorus atoms of the dippp ligand
and the two carbonyl ligands in-plane with the metal centre
and the hydride ligand located trans to the nitrosyl group.
Structurally related bond lengths of 2a are similar to those
of the chloride 1a.
22.9 Hz). The IR spectrum reveals strong bands at 2019,
1950 [ν(CO)] and 1610 cm–1 [ν(NO)]. X-ray diffraction
studies of 1a confirmed the spectroscopically derived
pseudo-octahedral structures. An ORTEP drawing of 1a is
presented in Figure 1.
Scheme 1.
Figure 2. ORTEP drawing of [Mo(dippp)(NO)(CO)2(H)] (2a). Dis-
placement ellipsoids are drawn at the 50% probability level. Se-
lected hydrogen atoms have been omitted for clarity. Selected bond
lengths [Å] and angles [°] for 2a: Mo1–N1 1.829(2), Mo1–P1
2.5569(6), Mo1–P2 2.5347(7), Mo1–C1 2.017(3), Mo1–C2 2.018(3),
Mo1–H 1.66(3), C1–O2 1.136(4), C2–O3 1.136(4), N1–O1 1.190(3),
P1–Mo1–P2 90.3(2), C1–Mo1–C2 88.1(1).
[30–32]
Treatment of 2a with 1 equiv. of [H(Et2O)2][BArF
]
4
[ArF = 3,5-(CF3)2C6H3] in THF afforded [Mo(dippp)-
(NO)(CO)2(THF)][BArF4] (3a; Scheme 1), which was iso-
lated in 69% yield. The 31P{1H} NMR spectrum of 3a
1
shows a singlet at δ = 17.6 ppm. In the H NMR spectrum
Figure 1. ORTEP drawing of [Mo(dippp)(NO)(CO)2(Cl)] (1a). Dis-
placement ellipsoids are drawn at the 50% probability level. All
characteristic multiplet resonances of THF appear at δ =
hydrogen atoms and the positional disorder of NO and Cl have 3.62 and 1.78 ppm with the integration value correlating to
integrations of the dippp ligand signals. The 13C{1H} NMR
been omitted for clarity. Selected bond lengths [Å] and angles [°]
for 1a: Mo1–P1 2.5576(4), Mo1–P2 2.5780(5), Mo1–C17 2.043(2),
spectrum reveals a carbonyl resonance at δ = 214.1 ppm
Mo1–C16 2.034(2), C17–O3 1.130(2), C16–O2 1.134(2), N1B–O1B
(2JCP = 16.7 Hz) and the IR spectrum of 3a shows strong
1.218(12), P1–Mo1–P2 90.13(1), C17–Mo1–C16 89.75(7).
bands at 2044, 1978 [ν(CO)] and 1682 [ν(NO)] cm–1.
The crystal structure of 1a reveals a disorder between the
The X-ray diffraction studies of 3a (Figure 3) showed a
Cl atom and the trans-nitrosyl group. The phosphorus structure with pseudo-octahedral coordination of the metal
atoms and the two carbon atoms of the carbonyl groups centre similar to the hydride and chloride complexes 1a and
Eur. J. Inorg. Chem. 2011, 652–659
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653

A. Dybov, O. Blacque, H. Berke
2a with the two phosphorus atoms of the dippp ligands and Table 1. Selected spectroscopic data for 1a–d and 2a–d.
FULL PAPER
the carbonyl ligands in-plane with the metal centre. The
average M–P bond length of 2.588(2) Å is slightly longer
than the corresponding bond lengths in 2a. The THF mole-
cule is coordinated to the metal centre with a M–O bond
δ(MoP) δ(MoH) δ(CO)
[ppm] [ppm] [ppm]
IR vibration frequency [cm–1]
ν(CO)
ν(MH) ν(NO)
1a
21.5
61.1
29.8
53.5
41.1
84.5
48.3
53.5
215.9 2019 1950
217.1 2020
2l6.5 2021
1610
1611
1610
1614
1570
1570
1574
1574
length of 2.241(4) Å. In the related complex [(μ-H)W2(CO)7- 1b
1949
1950
1948
1910
1910
1902
1920
(THF)2(NO)][33] one THF molecule is also located trans to
1c
1d
2l7.4 2022
the nitrosyl ligand.
2a
–2.28 226.0 1983
–3.44 226.8 1983
–1.20 226.2 1981
–3.25 227.1 1986
1653
1653
1637
1647
2b
2c
2d
Complexes 1d and 2b were additionally characterized by
X-ray diffraction analyses. The ORTEP drawings of 1d and
2b are shown in Figure 4. Similarly to 1a the structure of
1d displays a pseudo-octahedral coordination geometry
with two phosphane atoms and two carbonyls in-plane with
the molybdenum centre. The asymmetric unit of 1d con-
tains one half of the molecule as the metal centre lies on a
crystallographic two-fold axis. Consequently, the trans Cl
and NO ligands are positionally disordered with an occu-
pancy ratio of 0.5. The Mo–P bond length is 2.5319(7) Å,
which is slightly shorter than the corresponding mean value
of 1a, presumably due to the smaller bite angle of the dcype
ligand in comparison with the dippp derivative.[34] A similar
tendency can be observed on comparing the bond lengths
of 2a and 2b. The mean Mo–P bond length in 2b is 2.513 Å,
whereas the mean Mo–P bond length in 2a was found to
be 2.546 Å. The Mo1–H bond is rather long [1.83(4) Å],
which indicates the suitability of such a complex for hydride
transfer processes.
Figure 3. ORTEP drawing of [Mo(dippp)(NO)(CO)2(THF)]-
[BArF4] (3a). Displacement ellipsoids are drawn at the 50% prob-
ability level. Only the main residues (without counterion and free
solvent molecules) are shown. Hydrogen atoms have been omitted
for clarity. Selected bond lengths [Å] and angles [°] for 5a: Mo1–N1
1.780(6), Mo1–P1 2.582(2), Mo1–P2 2.5840(15), Mo1–O1 2.241(4),
Mo1–C6 2.053(8), Mo1–C5 2.060(7), C6–O4 1.12(1), C5–O2
1.12(1), N1–O3 1.188(7), P1–Mo1–P2 88.40(5), C1–Mo1–C2
87.6(3), O1–Mo1–N1 173.3(3).
The reaction of 2a with [H(Et2O)2][B(C6F5)4] produced
the cationic complex [Mo(dippp)(NO)(CO)2(THF)]-
[B(C6F5)4] (4a). The 31P{1H} NMR spectrum exhibits a sin-
glet resonance at δ = 16.5 ppm for the dippp ligand. The
13C{1H} NMR spectrum shows a doublet of doublets at δ
= 214.0 ppm (2JCP = 22.1 Hz) attributed to the carbonyl
ligands. The IR spectrum of 4a displays two strong ν(CO)
bands at 2038 and 1970 cm–1 and a ν(NO) band at
1668 cm–1. However, the elemental analysis did not match
the expected values presumably due to the fact that 4a is an
oil and could not be isolated in crystalline form. We sup-
posed that the substance contained an excess of THF, which
could not be removed in vacuo and prevented 4a from
crystallization.
A study of the influence of phosphane ligands on the
reactivity of the complexes was attempted by the prepara-
tion of a series of molybdenum hydrides with different
phosphane ligands (Scheme 1). The chlorides 1b–1d were
prepared by the reaction of [Mo(CO)4(NO)ClAlCl3] with
the diphosphane ligands 1,2-bis(diisopropylphosphanyl)-
ethane (dippe, b), 1,1Ј-bis(diisopropylphosphanyl)ferrocene
(dippf, c) and 1,2-bis(dicyclohexylphosphanyl)ethane
(dcype, d) in THF at room temperature. Then the chlorides
were treated with excess LiBH4 in Et3N to afford the hy-
dride complexes 2b–2d. The NMR and IR spectra reveal
the close structural relationships between 1b–1d and 1a and
Figure 4. ORTEP drawings of [Mo(dcype)(NO)(CO)2(Cl)] (1d) and
[Mo(dippe)(NO)(CO)2(H)] (2b). Displacement ellipsoids are drawn
at the 50% probability level. Selected hydrogen atoms and the posi-
tional disorder of NO and Cl in 1d have been omitted for clarity.
Selected bond lengths [Å] and angles [°] for 1d: Mo1–N1 1.772(5),
Mo1–P1 2.5319(7), Mo1–Cl1 2.517(3), Mo1–C1 2.043(2), C1–O1
1.135(4), N1–O2 1.195(6) Cl1B–Mo1–N1B 179.4(2), P1–Mo1–P1
81.23(2), C1–Mo1–C1 89.0(1). Selected bond lengths [Å] and
angles [°] for 2b: Mo1–N1 1.868(3), Mo1–P1 2.5054(8), Mo1–P2
2.5205(8), Mo1–H 1.83(4), Mo1–C1 2.007(3), Mo1–C2 2.023(3),
C1–O1 1.148(7), C2–O2 1.137(4), N1–O3 1.152(4), P1–Mo1–P2
80.26(3), C1–Mo1–C2 90.1(1).
Imine Hydrogenation
Complex 3a was first tested as a catalyst in the hydrogen-
between 2b–2d and 2a. The spectroscopic data for com- ation of the imine PhCH=N(α-naphthyl). The reaction was
pounds 1 and 2 are summarized in Table 1.
carried out at 10 bar H2 and 80 °C in C6H5Cl.[35] However,
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Molybdenum Nitrosyl Complexes
only 5% conversion of the imine was observed (Table 2). A exhibited significantly higher rates of hydrogenation (initial
kinetic analysis demonstrated that the reaction proceeded TOF values: 106 h–1 for 2a, 123 h–1 for 2b). Most likely, the
well during the initial 30 min after which the catalyst de- shorter bridge “pulls” the sterically demanding isopropyl
composed presumably due to the limited stability of the group to one side opening up another side of the molecule
[BArF4]– counterion.[36,37] We therefore looked for an alter- to allow the substrate enter.
native more stable counterion. The [B(C6F5)4]– anion is
The hydride 2b showed the best catalytic performance
known to display a lower coordination ability and higher of all the experiments with PhCH=N(α-naphthyl) and was
stability towards fluorine abstraction than [BArF4]–.[38]
therefore used in the hydrogenation of other imines. The
results of these catalytic experiments are presented in
Table 3.
Table 2. Catalytic hydrogenation of PhCH=N(α-naphthyl).
Table 3. Catalytic hydrogenation of imines with 2b as catalyst.
Entry Cat. Pressure
[bar]
Temp.
[°C]
Initial
Conv. Reaction time
TOF [h–1
]
[%]
[h]
1
2
3
4
5
6
3a
10
30
30
30
30
30
80
r.t.
r.t.
r.t.
r.t.
r.t.
–
106
95
123
68
50
5
0.5
13
14
11
16
19
2a[a]
4a
Ͼ99
Ͼ99
Ͼ99
Ͼ99
Ͼ99
Entry Substrate
TOF
[h–1]
Conv. [%] Reaction
time [h]
2b[a]
2c[a]
2d[a]
1
2
3
4
5
6
7
PhCH=N(α-naphthyl)
PhCH=NPh
p-ClC6H4CH=NPh
p-ClC6H4CH=N-p-C6H4Cl
PhCH=NCHPh2
PhCH=NMes
123
34
74
41
18
84
–
Ͼ99
80
11
24
16
17
35
20
–
96
66
[a] A mixture of the corresponding hydride and [H(Et2O)2][B-
(C6F5)4] was used as a catalyst.
45
Ͼ99
≈0.3
PhCH=NiBu
The use of 4a as the catalyst in the imine hydrogenation
of PhCH=N(α-naphthyl) revealed 100% conversion of the
substrate and a TOF value of 95 h–1 in the first hour at
A mechanism for the hydrogenation of imines with the
room temperature under a H2 pressure of 30 bar (Table 2). Mo catalysts is presented in Scheme 2. The initial proton-
An increase in temperature led to significantly lower con- ation of the hydride results in a short-lived dihydrogen com-
versions, presumably due to the decomposition of the cata- plex,[41] which is the key species that drives the catalytic
lyst during catalysis. Note that no hydrogenation was ob- cycle. An iminium salt might then be formed by proton
served when the reaction was conducted in THF as solvent. transfer from the acidic dihydrogen complex.[4,13] To vali-
Moreover, more quantitative experiments, in which THF date the iminium as a key intermediate in the hydrogena-
was initially added to the reaction mixture in various tion, a reduction was carried out in which a catalytic
amounts, demonstrated an inverse dependence of the rate amount of separately obtained iminium salt and 2b were
of hydrogenation on the amount of added THF. We sup- used. This experiment did not reveal any significant differ-
posed that THF acts as a ligand and thereby blocks catalyt- ence in the rate of hydrogenation and conversion compared
ically crucial vacant sites of various intermediates and con- with the result described above (Table 2, entry 1). Both the
sequently reduces the catalytic turnover. Hydrogenation re- imine and amine product are expected to be involved in a
actions without the involvement of THF were also realized protolytic equilibrium, which would require that the electro-
by the direct use of the pure hydride 2a and [H(Et2O)2]- philic iminium as a reactant is present in sufficient concen-
[B(C6F5)4] as a co-catalyst. Solutions of the substrate in tration that subsequent hydride transfer could become ki-
C6H5Cl were added to this mixture and pressurized with netically feasible. The deprotonation of the dihydrogen li-
30 bar of H2. This approach revealed a slightly higher rate gand comprises the heterolytic splitting of H2 and this pre-
of hydrogenation (TOF = 105 h–1) than with pure 4a.
sumably is the rate-limiting step. The highest rates of hydro-
To study the influence of the bite angle of the diphos- genation were observed with more bulky substrates (en-
phane ligands[39] on the reactivity the hydrides 2b–2d were tries 1 and 6, Table 3), which apparently facilitate the amine
also tested in the hydrogenation of PhCH=N(α-naphthyl) dissociation from the molybdenum centre required for re-
(Table 2). The lowest rate of hydrogenation was found for entry of H2 into the cycle. Amine, imine and H2 coordina-
2d (initial TOF = 50 h–1), presumably due to the high steric tion have to be considered competitive.
congestion of the cyclohexyl substituent of the dcype li-
Crucial to the course of the hydrogenation reaction is the
gand, which prevents a facile approach of the PhCH=N(α- formation of the iminium cation, which competes with the
naphthyl) substrate to the metal centre. Complexes 2a–2c formation of the ammonium salt produced from the pro-
showed a strong dependence of the diphosphane bite an- tonation of the amine products. All imine/amine pairs with
gle[40] on the rate of hydrogenation. Complex 2c with the aryl substituents generally showed low basicities, but the
largest bite angle in this series demonstrated the lowest ac- imine is the stronger base. Conversely, the basicities of
tivity (initial TOF = 68 h–1), whereas the hydrides 2a and 2b alkyl-substituted imine/amine are generally higher with the
Eur. J. Inorg. Chem. 2011, 652–659
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655

A. Dybov, O. Blacque, H. Berke
FULL PAPER
Scheme 2. Mechanism of imine hydrogenation following a proton-before-hydride transfer scheme.
amine more basic than the corresponding imine derivatives. is too unstable under the conditions employed in the imine
Thus, we anticipated that the hydrogenation of aniline de- hydrogenations. In contrast, in the presence of the
rivatives could be readily accomplished because their imines [B(C6F5)4]– anion the reactive 16e– molybdenum centre pos-
are stronger bases than the amines and the iminium cation sesses enough stability and flexibility to promote the proper
should therefore be available in kinetically relevant concen- exchanges of weak ligands. The complex [Mo(dippe)-
trations. In the case of alkylimine derivatives (entries 5 and (CO)2(NO)H] (2b) displayed the best activity with an initial
7, Table 3) we could not achieve satisfactory conversions TOF of 123 h–1 in the hydrogenation of PhCH=N(α-
probably due to the fact that the amine is the stronger base naphthyl). This hydride was also tested as a catalyst to hy-
and would consume a significant proportion of the protons drogenate various other aryl imines. The hydrogenation of
thus blocking their involvement in the catalytic cycle. The alkyl imines was found to be inferior to aryl imines. All
alkylamines would also be stronger ligands and could block these observations can be explained on the basis of an
the metal centre if not enough protons can be supplied to “ionic hydrogenation” pathway occurring with heterolytic
scavenge the amine as an ammonium salt. In the case of cleavage of H2 and “proton-before-hydride” transfer.
N-alkyl-substituted PhCH=NCHPh2, the basicities of the
amine and the imine are presumably comparable and the
reaction was found to stop when a certain amount of amine
is produced, which would be in accord with the given ratio-
General: Reagent-grade benzene, toluene, pentane, diethyl ether
nalization of the protolytic equilibrium.
The difference in pKb of the PhCH=NiBu/PhCH2NHiBu
pair seems to be so high that only the amine is expected
Experimental Section
and tetrahydrofuran were dried and distilled from sodium benzo-
phenone ketyl prior to use. Acetone and CH2Cl2 were dried with
CaH2 and then distilled. Literature procedures were used to pre-
pare the following compounds: 1,3-bis(diisopropylphosphanyl)pro-
pane (dippp),[43] 1,2-bis(diisopropylphosphanyl)ethane (dippe), 1,2-
to be present in the protonated form. Indeed, the rate of
hydrogenation of this compound was zero. Presumably the
low yield of the reaction of entry 4 of Table 3 with a chlo- bis(dicyclohexylphosphanyl)ethane (dcype),[44] 1,1Ј-bis(diphenyl-
phosphanyl)ferrocene (dippf),[45] [H(Et2O)2][BArF4],[46] [H(Et2O)2]-
rine-containing substrate can be related to a blocking of the
[B(C6F5)4][38] and [Mo(CO)4(NO)(ClAlCl3).[47] Other reagents were
molybdenum centre with chloride released during cataly-
sis.[42]
purchased and used without further purification. All the manipula-
tions were carried out under nitrogen using Schlenk techniques or
in a dry glovebox. IR spectra were obtained with a Bio-Rad FTS-
45 instrument. NMR spectra were recorded with Varian Mercury
Conclusions
200 (200.1 MHz for 1H, 81.0 MHz for 31P{1H}), Bruker DRX 500
(500.2 MHz for 1H, 202.5 MHz for 31P{1H}, 125.8 MHz for
A series of hydride complexes of the type [Mo(PʝP)-
(CO)2(NO)H] bearing bulky diphosphane ligands have been
prepared by the reaction of [Mo(PʝP)(CO)2(NO)Cl] with
LiBH4 in Et3N at room temperature. In combination with
[H(Et2O)2][B(C6F5)4] the hydrides showed valuable catalytic
activity in the hydrogenation of the imine PhCH=N(α-
13C{1H}) and Bruker DRX 400 spectrometers (400.1 MHz for H,
1
162.0 MHz for 31P{1H}, 100.6 MHz for 13C{1H}). Chemical shifts
in 1H and 13C{1H} NMR spectra are given in ppm relative to TMS
(SiMe4), the resonances in the 31P{1H} NMR spectra are refer-
enced to 98% external H3PO4. Elemental analyses were performed
with a Leco CHN(S)-932 instrument. The hydrogen pressure in the
naphthyl). The widely used [BArF4]– counterion apparently catalytic reactions was monitored with “WIKA” Transmitter CPT
656 Eur. J. Inorg. Chem. 2011, 652–659
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Molybdenum Nitrosyl Complexes
2500 instrument. The GC–MS spectra were recorded with a Varian
Saturn 2000 spectrometer equipped with a Varian 450-GC chroma-
tograph. Gas flow: 1.0 mL/min. Temperature regime: 70 °C,
0.5 min hold, then 20 °C/min. The following column was used: VF-
5ms 30.0 mϫ0.25 mm, ID = 0.25 mm.
General Procedure for the Synthesis of the Diphosphane-Substituted
Hydrides [Mo(PʝP)(CO)2(NO)H] (2a–2d): Et3N (10 mL) was
added to a mixture of [Mo(PʝP)(CO)2(NO)Cl] (0.25 mmol) and
LiBH4 (27.5 mg, 1.25 mmol). The reaction mixture was stirred at
room temperature until no trace of the starting material [monitor-
ing with 31P(1H) NMR] was observed (≈10 h). The Et3N was re-
moved in vacuo. The residue was extracted with benzene until the
extracted solution became colourless. After removal of the benzene
the residue was extracted again with hexane until the extracted
solution became colourless. Then the combined hexane fractions
were concentrated to half of their original volume and cooled to
–30 °C. The precipitate formed was filtered off and dried in vacuo.
General Procedure for the Synthesis of the Diphosphane-Substituted
Complexes [Mo(PʝP)(CO)2(NO)Cl] (1a–1d): A solution of the di-
phosphane ligand (1.0 mmol) in THF (10 mL) was added to a solu-
tion of [Mo(CO)4(NO)ClAlCl3] (1.0 mmol) in THF (15 mL). The
reaction mixture was stirred for 10 h at room temperature. Then
the solvent was removed in vacuo. The residue was extracted with
Et2O until the extracted solution became colourless. The combined
Et2O fractions were concentrated to the half of their original vol-
ume and cooled to –30 °C. The precipitate formed was filtered off
and dried in vacuo.
[Mo(dippp)(CO)2(NO)H] (2a). Yield 67%, 76.8 mg. 1H NMR
(400.1 MHz, C6D6, 25 °C): set of multiplets from 1.82 to 1.62 [4
H, PCH(CH3)2], set of multiplets from 1.62 to 1.28 (6 H,
PCH2CH2CH2P), set of multiplets from 1.15 to 0.79 (24 H, CH3),
[Mo(dippp)(CO)2(NO)Cl] (1a): Yield 75%, 372.2 mg. 1H NMR
(400.1 MHz, CD2Cl2, 25 °C): δ = 2.36 [m, 2 H, PCH(CH3)2], 2.27
[m, 2 H, PCH(CH3)2], 2.12 (m, 4 H, PCH2CH2CH2P), 1.90 (m, 8
H, PCH2CH2CH2P), set of multiplets from 1.33 to 1.25 (24 H,
CH3) ppm. 31P{1H} NMR (162.0 MHz, CD2Cl2, 25 °C): δ = 21.5
(s) ppm. 13C{1H} NMR (100.6 MHz, CD2Cl2, 25 °C): δ = 215.9
2
–2.28 (t, JPH = 24.0 Hz, 1 H, Mo-H) ppm. 31P{1H} NMR
(162.0 MHz, C6D6, 25 °C): δ = 41.1 (s) ppm. 13C{1H} NMR
2
(100.6 MHz, C6D6, 25 °C): δ = 226.0 (t, JCP = 10.7 Hz, CO), 29.2
1
1
[t, JCP = 10.7 Hz, PCH(CH3)2], 27.6 [t, JCP = 13.1 Hz, PCH-
2
1
(CH3)2], 22.7 (t, JCP = 4.8 Hz, PCH2CH2CH2P), 20.0 (t, JCP
=
2
2
9.5 Hz, PCH2CH2CH2P), 19.6 (s, CH3), 18.8 (s, CH3), 18.8 (s,
(dd, JCP = 38.2, JCP = 22.7 Hz, CO), 27.4 [m, PCH(CH3)2], 26.6
[m, PCH(CH3)2], 21.9 (m, PCH2CH2CH2P), 20.0 (m,
PCH2CH2CH2P), 19.9 (s, CH3), 19.7 (s, CH3), 19.1 (s, CH3) ppm.
CH ), 17.5 (s, CH ) ppm. IR (ATR): ν = 1983, 1910 (CO), 1653
˜
3
3
(MoH), 1570 (NO) cm–1. C17H35MoNO3P2 (459.4): calcd. C 44.45,
2019, 1950 (CO) 1610 (NO) cm–1.
H 7.68, N 3.05; found C 44.27, H 7.99, N 3.10.
IR (ATR):
ν
˜
=
[Mo(dippe)(CO)2(NO)H] (2b): Yield 53%, 59.4 mg. 1H NMR
(400.1 MHz, C6D6, 25 °C): δ = 1.78 [m, 2 H, PCH(CH3)2], 1.67 [m,
2 H, PCH(CH3)2], 1.18 (m, 4 H, PCH2CH2P), set of multiplets
C17H34ClMoNO3P2 (493.8): calcd. C 41.35, H 6.94, N 2.84; found
C 41.08, H 7.04, N 2.69.
[Mo(dippe)(CO)2(NO)Cl] (1b): Yield 43%, 205.9 mg. 1H NMR
(400.1 MHz, CD2Cl2, 25 °C): δ = 2.41 [m, 2 H, PCH(CH3)2], 2.30
[m, 2 H, PCH(CH3)2], 1.94 (m, 4 H, PCH2CH2P), set of multiplets
from 1.37 to 1.23 (24 H, CH3) ppm. 31P{1H} NMR (162.0 MHz,
CD2Cl2, 25 °C): δ = 61.1 (s) ppm. 13C{1H} NMR (100.6 MHz,
2
from 1.09 to 0.81 (24 H, CH3), 3.43 (t, JPH = 26.3 Hz, 1 H, Mo-
H) ppm. 31P{1H} NMR (162.0 MHz, C6D6, 25 °C): δ = 84.5
(s) ppm. 13C{1H} NMR (100.6 MHz, C6D6, 25 °C): δ = 226.8 (dd,
2
2JCP = 33.4, JCP = 10.7 Hz, CO), 28.3 [m, PCH(CH3)2], 26.7 [m,
1
2
2
PCH(CH3)2], 22.5 (t, JCP = 15.5 Hz, PCH2CH2P), 19.7 (s, CH3),
CD2Cl2, 25 °C): δ = 217.0 (dd, JCP = 51.3, JCP = 10.7 Hz, CO),
1
19.5 (s, CH ), 19.3 (s, CH ), 18.8 (s, CH ) ppm. IR (ATR): ν =
˜
3
3
3
25.6 [m, PCH(CH3)], 24.0 [m, PCH(CH3)2], 22.9 (dd, JCP = 17.9,
1983, 1910 (CO), 1653 (MoH), 1570 (NO) cm–1. C16H33MoNO3P2
(445.3): calcd. C 43.15, H 7.47, N 3.15; found C 42.71, H 7.79, N
3.18.
1JCP = 14.3 Hz, PCH2CH2P), 20.5 (s, CH3), 20.3 (s, CH3), 20.2 (s,
CH ), 18.6 (s, CH ) ppm. IR (ATR): ν = 2020, 1949 (CO), 1611
˜
3
3
(NO) cm–1. C16H32ClMoNO3P2 (479.8): calcd. C 40.05, H 6.72, N
[Mo(dippf)(CO)2(NO)H] (2c): Yield 49%, 74.6 mg. 1H NMR
(400.1 MHz, C6D6, 25 °C): δ = 4.17 (s, 2 H, Cp), 4.03 (s, 2 H, Cp),
4.01 (s, 2 H, Cp), 3.98 (s, 2 H, Cp), 2.12 [m, 4 H, PCH(CH3)2], set
2.92; found C 40.21, H 6.63, N 2.81.
[Mo(dippf)(CO)2Cl] (1c): Yield 64%, 406.8 mg. 1H NMR
(400.1 MHz, CD2Cl2, 25 °C): δ = 4.56 (s, 2 H, Cp), 4.45 (s, 2 H,
Cp), 4.44 (s, 2 H, Cp), 4.41 (s, 2 H, Cp), 2.55 [m, 4 H, PCH-
(CH3)2], set of multiplets from 1.55 to 1.23 (24 H, CH3) ppm.
31P{1H} NMR (162.0 MHz, CD2Cl2, 25 °C): δ = 29.8 (s) ppm.
2
of multiplets from 1.27 to 0.96 (24 H, CH3), –1.20 (t, JPH
=
23.6 Hz, 1 H, Mo-H) ppm. 31P{1H} NMR (162.0 MHz, C6D6,
25 °C): δ = 48.2 (s) ppm. 13C{1H} NMR (100.6 MHz, C6D6,
2
2
13C{1H} NMR (100.6 MHz, CD2Cl2, 25 °C): δ = 216.5 (dd, JCP
2
25 °C): δ = 226.2 (dd, JCP = 38.1, JCP = 11.0 Hz, CO), 76.9 [m,
2
= 61.2, 2JCP = 18.1 Hz, CO), 76.4 [m, PC(CH)2(CH)2], 75.6 [t, 2JCP
PC(CH)2(CH)2], 75.3 [t, JCP = 4.0 Hz, PC(CH)2(CH)2], 74.9 [t,
2JCP = 4.0 Hz, PC(CH)2(CH)2], 71.0 [t, JCP = 2.0 Hz, PC(CH)2-
2
2
= 5.0 Hz, PC(CH)2(CH)2] 75.3 [t, JCP = 4.0 Hz, PC(CH)2(CH)2],
2
1
2
2
(CH)2], 70.9 [t, JCP = 2.0 Hz, PC(CH)2(CH)2], 29.2 [dd, JCP
=
=
71.7 [t, JCP = 2.0 Hz, PC(CH)2(CH)2], 71.4 [t, JCP = 2.0 Hz,
1
1
1
1
PC(CH)2(CH)2], 29.2 [t, JCP = 8.03 Hz, PCH(CH3)2], 28.7 [t, 1JCP
11.0, JCP = 9.0 Hz, PCH(CH3)2], 26.5 [dd, JCP = 12.1, JCP
10.0 Hz, PCH(CH3)2], 20.0 (s, CH3), 19.8 (s, CH3), 19.1 (s, CH3),
= 9.0 Hz, PCH(CH3)2], 20.9 (s, CH3), 20.5 (s, CH3), 19.8 (s, CH3),
19.1 (s, CH ) ppm. IR (ATR): ν = 1983, 1902 (CO), 1637 (MoH),
˜
18.8 (s, CH ) ppm. IR (ATR): ν = 2021, 1950 (CO), 1610
˜
3
3
1574 (NO) cm–1. C24H37FeMoNO3P2 (601.3): calcd. C 47.94, H
(NO) cm–1. C24H36ClFeMoNO3P2 (635.7): calcd. C 45.34, H 5.71,
6.20, N 2.33; found C 47.80, H 6.11, N 2.27.
N 2.20; found C 45.23, H 5.80, N 2.15.
[Mo(dcype)(CO)2(NO)Cl] (1d): Yield 83%, 265.6 mg. 1H NMR
[Mo(dcype)(CO)2(NO)H] (2d): Yield 63%, 95.6 mg. 1H NMR
(500.2 MHz, CD2Cl2, 25 °C): set of multiplets from 2.2 to 1.1.
(400.1 MHz, C6D6, 25 °C): set of multiplets from 2.2 to 1.1, –3.25
2
31P{1H} NMR (202.5 MHz, CD2Cl2 25 °C): δ = 53.5 (s) ppm. (t, JPH = 26.3 Hz, 1 H, Mo-H) ppm. 31P{1H} NMR (162.0 MHz,
2
13C{1H} NMR (160.5 MHz, CD2Cl2, 25 °C): δ = 217.4 (dd, JCP C6D6, 25 °C): δ = 53.5 (s) ppm. 13C{1H} NMR (100.6 MHz, C6D6,
2
2
2
= 51.3, JCP = 10.7 Hz, CO), 35.9 (m, PCH), 34.0 (m, PCH), 30.7
25 °C): δ = 227.1 (dd, JCP = 32.2, JCP = 10.7 Hz, CO), 38.0 (m,
(s, Cy), 30.6 (s, Cy), 30.3 (s, Cy), 29.1 (s, Cy), 28.1 (m, Cy), 27.8
PCH), 37.0 (m, PCH), 29.9 (s, Cy), 29.8 (s, Cy), 29.2 (s, Cy), 28.9
1
1
(m, Cy), 27.7 (m, Cy), 26.7 (s, Cy), 26.6 (s, Cy), 21.0 (dd, JCP
=
(s, Cy), 27.8 (m, Cy), 26.8 (s, Cy), 26.6 (s, Cy), 22.6 (dd, JCP
=
17.9, 1JCP = 14.3 Hz, PCH CH P) ppm. IR (ATR): ν = 2022, 1948
17.6, 1JCP = 15.5 Hz, PCH CH P) ppm. IR (ATR): ν = 1986, 1920
˜
2 2
˜
2
2
(CO), 1614 (NO) cm–1. C28H48ClMoNO3P2 (640.0): calcd. C 52.54, (CO), 1647 (MoH), 1574 (NO) cm–1. C28H49MoNO3P2 (605.6):
H 7.56, N 2.19; found C 52.38, H 7.47, N 2.15. calcd. C 55.53, H 8.16, N 2.31; found C 55.17, H 8.34, N 2.26.
Eur. J. Inorg. Chem. 2011, 652–659
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
657

A. Dybov, O. Blacque, H. Berke
Synthesis of [Mo(dippp)(CO)2(NO)(THF)][BArF4] (3a): A solution 8.491 min, m/z = 225; PhCH=NiBu: tR = 4.876 min, m/z = 161;
FULL PAPER
of [H(Et2O)2][BArF4] (43.8 mg, 0.043 mmol) in THF (2 mL) was
slowly added to a solution of [Mo(dippp)(CO)2(NO)H] (20.0 mg,
0.043 mmol) in THF (2 mL) at room temperature. The reaction
mixture was stirred for 2 h. Then the solvent was evaporated in
vacuo. Hexane (2 mL) was added to the residue and then the Et2O
was added dropwise to the suspension until the solid was com-
pletely dissolved. The solution was cooled to –30 °C to afford a
precipitate of 3a that was filtered and dried in vacuo. Yield 69%,
41.4 mg. 1H NMR (400.1 MHz, [D8]THF, 25 °C): δ = 7.79 (s, 8 H,
o-Ph), 7.57 (s, 4 H, p-Ph), 3.62 (m, 4 H, THF), 2.48 [m, 4 H,
PCH(CH3)2], 2.23 (m, 4 H, PCH2CH2CH2P), 1.91 (m, 2 H,
PCH2CH2CH2P), 1.77 (m, 4 H, THF), set of multiplets from 1.39
to 1.28 (24 H, CH3) ppm. 31P{1H} NMR (162.0 MHz, [D8]THF,
25 °C): δ = 17.7 (s) ppm. 13C{1H} NMR (100.6 MHz, [D8]THF,
PhCH2NHiBu: tR = 5.756 min, m/z = 163.
X-ray Diffraction Studies of 1a, 1d, 2a, 2b and 3a: Relevant details
regarding the structural refinements are given in Tables S1 and S2
of the Supporting Information and selected geometrical parameters
are included in the captions of the corresponding figures Intensity
data were collected at 183(2) K with an Oxford Xcalibur dif-
fractometer (4-circle kappa platform, Ruby CCD detector and a
single wavelength Enhance X-ray source with Mo-Kα radiation, λ =
0.71073 Å).[48] Suitable selected single crystals were mounted using
polybutene oil on the top of a glass fibre fixed on a goniometer
head and immediately transferred to the diffractometer. Pre-experi-
ment, data collection, data reduction and absorption corrections
were performed with CrysAlisPro.[48] The crystal structures were
solved with SHELXS-97[49] using direct methods. The structure re-
finements were performed by full-matrix least-squares methods on
F2 with SHELXL-97.[49] PLATON[50] was used to check the results
of the X-ray analyses. All programs used during the crystal struc-
ture determination process are included in the WINGX soft-
ware.[51] The hydride atoms were located in difference Fourier maps
and all other hydrogen atoms were placed at ideal positions and
refined with fixed isotropic displacement parameters using a riding
model.
2
2
25 °C): δ = 214.1 (dd, JCP = 32.2, JCP = 16.7 Hz, CO), 163.0 (q,
1JBC = 50.0 Hz, i-BArF4), 135.8 (s, o-BArF4), 130.1 (m, m-BArF4),
125.5 (q, 1JCF = 273.4 Hz, CF3), 118.4 (s, p-BArF4), 68.2 (s, THF),
2
27.9 [m, PCH(CH3)2], 26.4 (s, THF), 22.5 (t, JCP = 8.3 Hz,
PCH2CH2CH2P), 20.4 (s, CH3), 20.1 (s, PCH2CH2CH2P), 19.4 (s,
CH ), 19.1 (s, CH ), 18.7 (s, CH ) ppm. IR (ATR): ν = 2044, 1978
˜
3
3
3
(CO), 1682 (NO) cm–1. C53H54BF24MoNO4P2 (1393.7): calcd. C
45.68, H 3.91, N 1.01; found C 45.61, H 3.78, N 1.00.
CCDC-791648 (for 1a), -791649 (for 1d), -791650 (for 2a), -791651
(for 2b) and -791652 (for 3a) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/ data_request/cif.
Synthesis of [Mo(dippp)(CO)2(NO)(THF)][B(C6F5)4] (4a): A solu-
tion of [H(Et2O)2][B(C6F5)4] (37.1 mg, 0.043 mmol) in THF (2 mL)
was slowly added to a solution of [Mo(NO)(dippp)(CO)2H]
(20.0 mg, 0.043 mmol) in THF (2 mL) at room temperature. The
reaction mixture was stirred for 2 h. Then the solvent was evapo-
rated in vacuo to afford an oil that was used without further purifi-
Supporting Information (see footnote on the first page of this arti-
cle): X-ray crystallographic data for all determined molecular struc-
tures.
1
cation. H NMR (400.1 MHz, [D8]THF, 25 °C): δ = 3.37 (m, 4 H,
THF), 2.59 [m, 4 H, PCH(CH3)2], 2.23 (m, 4 H, PCH2CH2CH2P),
1.91 (m, 2 H, PCH2CH2CH2P), 1.77 (m, 4 H, THF), set of mul-
tiplets from 1.38 to 1.28 (24 H, CH3) ppm. 31P{1H} NMR
(162.0 MHz, [D8]THF, 25 °C): δ = 16.5 (s) ppm. 13C{1H} NMR
Acknowledgments
2
2
(125.8 MHz, [D8]THF, 25 °C): δ = 214.0 (dd, JCP = 32.1, JCP
=
1
22.1 Hz, CO), 149.0 (br. d, JCF = 235.9 Hz, o-C6F5), 139.1 (dm,
We thank the University of Zürich and the Swiss National Science
Foundation for financial support.
1JCF = 242.1 Hz, p-C6F5), 137.0 (dm, JCF = 244.1 Hz, m-C6F5),
1
125.7 (br. m, i-C6F5), 68.10 (s, THF), 28.0 [m, PCH(CH3)], 26.0 (s,
THF), 22.6 (t, 2JCP = 8.0 Hz, PCH2CH2CH2P), 20.5 (s, CH3), 20.2
(s, PCH2CH2CH2P), 19.5 (s, CH3), 19.2 (s, CH3), 18.8 (s,
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CH ) ppm. IR (ATR): ν = 2038, 1970 (CO), 1668 (NO) cm–1. The
˜
3
elemental analysis did not match the theoretical values due to pres-
ence of THF in a non-stoichiometric ratio.
General Procedure for the Catalytic Experiments: A solution of the
substrate (2.0 mmol) in C6H5Cl (5 mL) was added to a mixture of
the [Mo(PʝP)(CO)2(NO)H] catalyst (0.006 mmol, 0.3 mol-%) and
[H(Et2O)2][B(C6F5)4] (5.0 mg, 0.006 mmol). The mixture was
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pressure was monitored by a precision pressure probe. When the
pressure changes were approaching zero, the reaction mixture was
removed from the autoclave and filtered through silica gel. Without
any further purification, a GC–MS spectrum was recorded to de-
termine the conversion of the hydrogenation reaction and identify
the products. PhCH=N(α-naphthyl): tR = 10.532 min, m/z = 231;
PhCH2NH(α-naphthyl): tR = 10.607 min, m/z = 233; PhCH=NPh:
tR = 7.460 min, m/z = 181; PhCH2NHPh: tR = 7.695 min, m/z =
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183; p-ClC6H4CH=NPh: tR
= 8.636 min, m/z = 215; p-
ClC6H4CH2NHPh: tR = 8.989 min, m/z = 217; p-ClC6H4CH=N-p-
C6H4Cl: tR = 9.639 min, m/z = 249; p-ClC6H4CH2NH-p-C6H4Cl:
tR = 10.114 min, m/z = 251; PhCH=NCHPh2: tR = 10.589 min, m/z
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Wiley-VCH, Weinheim, 2007.
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=
271; PhCH2NHCHPh2: tR
= 10.386 min, m/z = 273;
PhCH=NMes: tR = 8.301 min, m/z = 223; PhCH2NHMes: tR
=
658
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 652–659

Molybdenum Nitrosyl Complexes
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Published Online: December 29, 2010
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